Hitherto, many processes have been developed for producing olefin polymers using metallocene complexes (e.g., JP-A-58-19309). It is reported that a complex having a ligand which comprises a phenoxy group and a cyclopentadienyl group crosslinked with each other through a bonding group, for example, dimethylsilyl(tetramethyl-cyclopentadienyl)(3-tert-butyl-5-methyl-2-phenoxy)titanium dichloride, is activated by a co-catalyst to show high catalytic activity for polymerizing an α-olefin, so that high molecular weight polyolefin can be produced (cf. JP-A-9-87313 and JP-A-2003-176295). However, there is a demand for an olefin polymerization catalyst capable of providing a high molecular weight polyolefin with still higher polymerization activity.